Nomerci: "Evil has to exist" - means is it a choice? :) I don't think so, it always will exist and so does the "goodness".

These two have co-existed since the beginning and barely have an effect on each other. However, things tend to change when they come across one another. The resultant is what matters.

So the real discussion should be that what should be done to bring the resultant to "Goodness"